The present invention relates to a developing unit driving means for a color image forming apparatus in which a plurality of developing units are disposed around an image forming member, and a color toner image is formed when a plurality of monochrome toner images are superimposed on each other.
Conventionally, as disclosed in Japanese Patent Publication Open to Public Inspection Nos. 257143/1987 and 226266/1990, the color image forming apparatus is structured as follows. A developing unit holder including a plurality of developing units is provided around an image forming member having a small diameter; the developing unit holder is rotated or slid so that a required developing unit can be selected when it is moved to a position opposed to the image forming member; a developing unit driving gear driven by a developing unit driving motor is previously disposed at a position opposed to the image forming member; and when the developing unit is moved to that position, the developing unit is automatically engaged with the gear.
However, in the development system in which the developing unit is integrally rotated with the developing unit holder as described above, the following disadvantages are encountered:
1 Because the developing unit itself, in which developer is accommodated, is rotated, the developer spills from the developing unit. In order to prevent the developer from spilling, it is necessary to provide a more complicated mechanism. PA1 2 Because the developing unit is always moved, it is necessary to provide a special mechanism by which the developing unit and the image forming member are very accurately positioned to each other every time when the developing unit is moved. PA1 3 Dimensions of a rotation mechanism, by which a plurality of developing units are integrally rotated, are increased, and therefore, dimensions of the apparatus are also increased. PA1 1 It is necessary to provide a plurality of electromagnetic clutches corresponding to the number of developing units, resulting in a higher cost and a larger space. PA1 2 In these clutches, in which clutch members are engaged with each other by the frictional force, because clutch members are worn out with the lapse of time, so that slippage occurs, the rotational speed of the developing sleeve is lowered. Accordingly, the development density is directly affected, and there is a possibility that the density is lowered.
On the other hand, there is provided a method in which a plurality of developing units are provided around the image forming member as disclosed in Japanese Patent Publication Open to Public Inspection No. 87770/1991, and a developing unit drive system of this development system is structured as follows. A drive transmission path from the developing unit driving motor, which is a driving source, to each developing unit is composed of a driving belt and a gear train. A plurality of electromagnetic clutches are disposed along the drive transmission path corresponding to each developing unit. When a required developing unit is rotated, the corresponding electromagnetic clutch is connected to the developing unit.
However, in this method, the following disadvantages are encountered:
The present invention has solved the foregoing problems, and the object of the present invention is to provide an inexpensive developing unit driving means, which can be installed into a smaller space, and is highly reliable.